/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package javax.servlet;

import java.io.IOException;

/**
 * Defines methods that all servlets must implement.
 * <p>
 * <p>
 * A servlet is a small Java program that runs within a Web server. Servlets
 * receive and respond to requests from Web clients, usually across HTTP, the
 * HyperText Transfer Protocol.
 * <p>
 * <p>
 * To implement this interface, you can write a generic servlet that extends
 * <code>javax.servlet.GenericServlet</code> or an HTTP servlet that extends
 * <code>javax.servlet.http.HttpServlet</code>.
 * <p>
 * <p>
 * This interface defines methods to initialize a servlet, to service requests,
 * and to remove a servlet from the server. These are known as life-cycle
 * methods and are called in the following sequence:
 * <ol>
 * <li>The servlet is constructed, then initialized with the <code>init</code>
 * method.
 * <li>Any calls from clients to the <code>service</code> method are handled.
 * <li>The servlet is taken out of service, then destroyed with the
 * <code>destroy</code> method, then garbage collected and finalized.
 * </ol>
 * <p>
 * <p>
 * In addition to the life-cycle methods, this interface provides the
 * <code>getServletConfig</code> method, which the servlet can use to get any
 * startup information, and the <code>getServletInfo</code> method, which allows
 * the servlet to return basic information about itself, such as author,
 * version, and copyright.
 *
 * @see GenericServlet
 * @see javax.servlet.http.HttpServlet
 */
public interface Servlet {

	/**
	 * Called by the servlet container to indicate to a servlet that the servlet
	 * is being placed into service.
	 * <p>
	 * <p>
	 * The servlet container calls the <code>init</code> method exactly once
	 * after instantiating the servlet. The <code>init</code> method must
	 * complete successfully before the servlet can receive any requests.
	 * <p>
	 * <p>
	 * The servlet container cannot place the servlet into service if the
	 * <code>init</code> method
	 * <ol>
	 * <li>Throws a <code>ServletException</code>
	 * <li>Does not return within a time period defined by the Web server
	 * </ol>
	 *
	 * @param config a <code>ServletConfig</code> object containing the servlet's
	 *               configuration and initialization parameters
	 * @throws ServletException if an exception has occurred that interferes with the
	 *                          servlet's normal operation
	 * @see UnavailableException
	 * @see #getServletConfig
	 */
	public void init(ServletConfig config) throws ServletException;

	/**
	 * Returns a {@link ServletConfig} object, which contains initialization and
	 * startup parameters for this servlet. The <code>ServletConfig</code>
	 * object returned is the one passed to the <code>init</code> method.
	 * <p>
	 * <p>
	 * Implementations of this interface are responsible for storing the
	 * <code>ServletConfig</code> object so that this method can return it. The
	 * {@link GenericServlet} class, which implements this interface, already
	 * does this.
	 *
	 * @return the <code>ServletConfig</code> object that initializes this
	 * servlet
	 * @see #init
	 */
	public ServletConfig getServletConfig();

	/**
	 * Called by the servlet container to allow the servlet to respond to a
	 * request.
	 * <p>
	 * <p>
	 * This method is only called after the servlet's <code>init()</code> method
	 * has completed successfully.
	 * <p>
	 * <p>
	 * The status code of the response always should be set for a servlet that
	 * throws or sends an error.
	 * <p>
	 * <p>
	 * <p>
	 * Servlets typically run inside multithreaded servlet containers that can
	 * handle multiple requests concurrently. Developers must be aware to
	 * synchronize access to any shared resources such as files, network
	 * connections, and as well as the servlet's class and instance variables.
	 * More information on multithreaded programming in Java is available in <a
	 * href
	 * ="http://java.sun.com/Series/Tutorial/java/threads/multithreaded.html">
	 * the Java tutorial on multi-threaded programming</a>.
	 *
	 * @param req the <code>ServletRequest</code> object that contains the
	 *            client's request
	 * @param res the <code>ServletResponse</code> object that contains the
	 *            servlet's response
	 * @throws ServletException if an exception occurs that interferes with the servlet's
	 *                          normal operation
	 * @throws IOException      if an input or output exception occurs
	 */
	public void service(ServletRequest req, ServletResponse res)
			throws ServletException, IOException;

	/**
	 * Returns information about the servlet, such as author, version, and
	 * copyright.
	 * <p>
	 * <p>
	 * The string that this method returns should be plain text and not markup
	 * of any kind (such as HTML, XML, etc.).
	 *
	 * @return a <code>String</code> containing servlet information
	 */
	public String getServletInfo();

	/**
	 * Called by the servlet container to indicate to a servlet that the servlet
	 * is being taken out of service. This method is only called once all
	 * threads within the servlet's <code>service</code> method have exited or
	 * after a timeout period has passed. After the servlet container calls this
	 * method, it will not call the <code>service</code> method again on this
	 * servlet.
	 * <p>
	 * <p>
	 * This method gives the servlet an opportunity to clean up any resources
	 * that are being held (for example, memory, file handles, threads) and make
	 * sure that any persistent state is synchronized with the servlet's current
	 * state in memory.
	 */
	public void destroy();
}
